The Department of Homeland Security has lost track of more than 6,000 foreign nationals who entered the United States on student visas, overstayed their welcome, and essentially vanished -- exploiting a security gap that was supposed to be fixed after the Sept. 11, 2001 terror attacks.

"My greatest concern is that they could be doing anything," said Peter Edge, the U.S. Immigration and Customs Enforcement official who oversees investigations into visa violators. "Some of them could be here to do us harm."

Homeland Security officials disclosed the breadth of the student visa problem in response to ABC News questions submitted as part of an investigation into persistent complaints about the nation’s entry program for students.

ABC News found that immigration officials have struggled to keep track of the rapidly increasing numbers of foreign students coming to the U.S. -- now in excess of one million each year. The immigration agency’s own figures show that 58,000 students overstayed their visas in the past year. Of those, 6,000 were referred to agents for follow-up because they were determined to be of heightened concern.

“They just disappear,” said Sen. Tom Coburn, R-Okla. “They get the visas and they disappear.”

Coburn said since the Sept. 11, 2001 terror attacks, 26 student visa holders have been arrested in the U.S. on terror-related charges.

Tightening up the student visa program was one of the major recommendations made by the 9/11 Commission, after it was determined that the hijacker who flew Flight 77 into the Pentagon, Hani Hanjour, had entered the U.S. on a student visa but never showed up for school.

Edge said ICE agents are trying to locate every one of the 6,000 missing students, but acknowledged that “we really have a lot more work to do” to tighten up the student visa program.

Despite repeated concerns raised by Congress, federal immigration officials have also continued to grant schools certification to accept overseas applicants even if the schools lack accreditation, state certification, or any obvious measure of academic rigor.

There are now more than 9,000 schools on the government approved list. The list includes such top flight American colleges as Harvard and Yale, but it also includes 86 beauty schools, 36 massage schools and nine schools that teach horseshoeing. Foreign students can enter the U.S. on a visa to study acupuncture, hair braiding, or join academies that focus on tennis and golf.

Once the student arrives in the U.S., it is up to the schools to keep track of the visa-holder’s whereabouts -- and report to the government if they repeatedly miss class.

That is a serious concern, Coburn said, because a number of for-profit schools appear to have been operating with a primary goal of selling visas, not educating students.

“We know we have a lot of non-accredited universities that are using this system to bring people in, collect money, and not educate them at all,” said Coburn, who is part of a bi-partisan group of senators that has been trying to tighten controls on student visas. “To me, it’s a mess.”